This application claims the benefit of priority from EPO Application No. 00201653.3 filed May, 9, 2000.
The invention relates to a method of recording at a recording speed an information signal on an information layer of a recording medium by irradiating the information layer by means of a radiation beam, said information layer having a phase reversibly changeable between a first state and a second state, which method comprises a write step for applying the radiation beam, in response to the information signal, to a first area of the information layer to cause the first area of the information layer to assume the first state, thereby forming a mark, and an erase step for applying a pulsed radiation beam to a second area of the information layer, before and after the mark, to cause the second area of the information layer to assume a state substantially identical to the second state, the pulsed radiation beam comprising erase pulses having an erase power level (Pe) and a bias power level (Pb) between the erase pulses.
The invention also relates to a recording device for recording at a recording speed an information signal on an information layer of a recording medium by irradiating the information layer by means of a radiation beam, said information layer having a phase reversibly changeable between a first state and a second state, the device comprising a radiation source for providing the radiation beam and control means for controlling the power of the radiation beam such that in response to the information signal the radiation beam is applied to a first area of the information layer to cause the first area of the information layer to assume the first state, thereby forming a mark, and such that a pulsed radiation beam comprising erase pulses having an erase power level (Pe) and a bias power level (Pb) between the erase pulses is applied to a second area of the information layer, before and after the mark, to cause the second area of the information layer to assume a state substantially identical to the second state.
A recording method of the kind set forth in the preamble is known from U.S. Pat. No. 5,291,470. A mark is written by a sequence of write pulses and the previously written marks between the marks being written are erased by applying a sequence of erase pulses. The write pluses are at a write power level (Pw) and the erase pulse are at an erase power level (Pe), the erase power level (Pe) being lower than the write power level (Pw). The power level between the write pulses as well as the erase pulses is a bias power level (Pb). In general, this bias power level (Pb) is set equal to the reading power level (Po).
The above method is known as a xe2x80x9cpulsed erasexe2x80x9d recording method as opposed to a recording method where the previously written marks between the marks being written are erased by applying a radiation beam having a constant erase power level. Because of this constant erase power level, the temperature of the information layer rises gradually. This temperature rise leads to erasure of not only marks in the actual track, but also of marks in adjacent tracks. This so-called xe2x80x9cthermal cross-writexe2x80x9d effect is reduced by applying the pulsed erase method.
It is a drawback of the xe2x80x9cpulsed erasexe2x80x9d recording method that it does not achieve sufficiently low jitter in the read signal obtained from reading marks recorded by using the xe2x80x9cpulsed erasexe2x80x9d method, especially when the marks are recorded at a variable recording speed. The jitter is the standard deviation of the time differences between level transitions in a digitized read signal and the corresponding transitions in a clock signal, the time difference being normalized by the duration of one period of said clock.
The recording speed may vary, for example, when recording at a Constant Angular Velocity (CAV). In this case the recording speed, i.e. the speed at which the recording medium moves relative to the spot of the radiation beam on the recording medium, increases when going from the center towards the periphery of a disc shaped recording medium. The recording speed is also known as the Linear Velocity.
It is an object of the invention to provide a method according to the opening paragraph for recording marks with reduced jitter while preventing xe2x80x9cthermal cross-writexe2x80x9d during erase.
This object is achieved by a method of the kind set forth which is characterized in that the bias power level (Pb) depends on the recording speed.
When the bias power level (Pb) is set at a fixed level no thermal cross write occurs while a minimal jitter of the recorded marks is obtained at a specific recording speed only. However, when the bias power level (Pb) is set in dependence on the recording speed, minimal jitter of the recorded marks may be obtained for each recording speed while still preventing the occurrence of thermal cross write to occur.
A more optimum setting can be obtained by introducing additional degree of freedom in the settings for the sequence of erase pulses (i.e., a variable bias power level instead of a fixed one), resulting in a reduced jitter of the recorded marks.
A version of the method according to the invention is characterized in that the bias power level (Pb) increases in a range between zero and the erase power level (Pe) as the recording speed (V) increases.
During recording at a low recording speed the spot of the radiation beam on the recording medium will remain comparatively long in one specific area of the recording medium compared to recording taking place at a high recording speed. Therefore, during recording at a low recording speed the area on the recording medium will experience a significant temperature rise resulting in a significant thermal cross-write. This temperature rise, and hence the thermal cross write, is reduced when a low bias power level (Pb) is used. However, during recording at a high recording speed the spot of the radiation beam on the recording medium will remain in one specific area of the recording medium for a comparatively short time only. Therefore, a higher bias power level (Pb), even up to the erase power level (Pe), may be used without introducing thermal cross write. Application of a high bias power level (Pb) results in low jitter of the recorded marks.
A preferred version of the method according to the invention is characterized in that the bias power level (Pb) increases in a range between zero and the erase power level (Pe) as the recording speed (V) increases when the recording speed is below a chosen recording speed, and that the bias power level (Pb) is substantially identical to the erase power level (Pe) when the recording speed exceeds the chosen recording speed.
Beyond a certain recording speed no thermal cross write occurs, because the spot of the radiation beam on the recording medium remains only very briefly in one specific area of the recording medium. Beyond this recording speed the bias power level (Pe) is set substantially equal to the erase power level (Pe), resulting in a radiation beam having a constant erase power level. Because of this constant erase power level a. Jitter of the recorded marks is minimized.
A version of the method according to the invention is characterized in that the duty cycle of the erase pulses depends on the recording speed.
In addition to changing the bias power level (Pb), the amount of energy transferred from the radiation beam to the recording medium, and hence the local temperature rise of the recording medium, can also be influenced by changing the duty cycle of the erase pulses. Both jitter of the recorded marks and thermal cross can both be minimized by setting the duty cycle of the erase pulses in dependency of the recording speed.
A version of the method according to the invention is characterized in that the duty cycle increases in a range between nearly zero and unity as the recording speed (V) increases.
During recording at a low recording speed, the spot of the radiation beam on the recording medium will remain comparatively long in one specific area of the recording medium compared to when recording takes place at a high recording speed. Therefore, during recording at a low recording speed the area on the recording medium will experience a significant temperature rise resulting in a significant thermal cross-write. This temperature rise, and hence the thermal cross write, is reduced when a smaller cycle of the erase pulses is used. However, during recording at a high recording speed, the spot of the radiation beam on the recording medium will remain in one specific area of the recording medium for a comparatively short time. Therefore, a higher duty cycle of the erase pulses, even up to the unity, may be used without giving rise to thermal cross write.
It is to be noted that the above methods according to the invention offer especially good results when used in combination with a recording medium having an information layer where the recrystallization process of the materials is determined by growth from the edges of amorphous marks. These so-called Fast Growth Materials (FGM) allow for high recording speeds but are sensitive to thermal cross write.
It is a further object of the invention to provide a recording device of the kind described in the opening paragraph which is arranged to carry out the method according to the invention.
This object is achieved by a recording device of the kind set forth in the preamble which is characterized in that the control means set the bias power level (Pb) in dependence on the recording speed (V).
A preferred embodiment of the recording device according to the invention is characterized in that the control means set the bias power level (Pb) so that it increases in a range between zero and the erase power level (Pe) as the recording speed (V) increases when the recording speed is below a chosen recording speed and that the control means set the bias power level (Pb) so as to be substantially identical to the erase power level (Pe) when the recording speed is above the chosen recording speed.